1. Field of the Invention
The present invention relates to methods for heating a metal member by laser light, methods for bonding heated metal members, and apparatuses for heating a metal member.
2. Description of the Related Art
Conventionally, there is a technique of heating a metal member by irradiating the surface of the metal member with laser light and causing the metal member to absorb the laser light. See, e.g., Japanese Patent No. 4894528 and No. 5602050, and Japanese Patent Application Publication No. 2014-228478 (JP 2014-228478 A).
Metal members are heated for various purposes. One of the purposes is to bond two members, as described in, e.g., Japanese Patent No. 4894528 and No. 5602050. For example, in the case of bonding two members, a metal member (e.g., a lead wire) that serves as a contact of an electric circuit is heated to directly bond the metal member to a member to which the metal member is to be bonded (e.g., a terminal of a semiconductor device). In this case, as described in Japanese Patent No. 4894528 and No. 5602050, a portion to be heated may not be heated until it forms a liquid phase. Namely, heating may be stopped while the portion is still in a solid phase, and the metal member and the member to which the metal member is to be bonded may be pressed and bonded together with a predetermined pressure (solid-phase diffusion bonding). Alternatively, these members may be bonded by normal welding. Namely, the portion to be heated may be melted into a liquid phase, and the metal member and the member to which the metal member is to be bonded may be bonded together. This bonding is more resistant to high temperature environments than in the case where the metal member and the member to which the metal member is to be bonded are bonded by, e.g., solder.
Another purpose of heating is to nondestructively examine if a metal member and a member having the metal member bonded thereto have been bonded with a sufficient contact area, as described in, e.g., JP 2014-228478 A. In the technique of JP 2014-228478 A, the metal member bonded to the member is irradiated with laser light to heat the metal member, so that the temperature of the metal member rises. If the metal member and the member have been bonded with a sufficient contact area, the heat is satisfactorily transferred from the metal member to the member according to the contact area, and the temperature of the metal member therefore rises slowly. However, if the metal member and the member have been insufficiently bonded with an insufficient contact area, the heat of the metal member cannot be satisfactorily transferred to the member, and the temperature of the metal member therefore rises sharply. The bonded state between the metal member and the member is evaluated by this difference in rate of temperature rise.
In the above description, an inexpensive yttrium aluminum garnet (YAG) laser etc. is often used to emit laser light. YAG lasers are lasers that emit near-infrared laser light (0.7 μm to 2.5 μm). For example, YAG laser absorption of metal members made of copper or aluminum is very low at low temperatures up to a predetermined temperature (e.g., a melting point). Accordingly, if the metal member described in the technique of Japanese Patent No. 4894528, Japanese Patent No. 5602050, or JP 2014-228478 A is made of copper or aluminum, even if the metal member is directly irradiated with laser light, the temperature of the metal member rises slowly in a low temperature range due to its low laser absorption. A large amount of energy is therefore consumed until the temperature of the metal member reaches the predetermined temperature at which the laser absorption of the metal member increases.
As a solution to this, in the technique described in JP 2014-228478 A, laser absorption of the metal member at low temperatures is enhanced by forming an oxide film on the surface of the metal member, based on the known knowledge. The oxide film is formed by irradiating the surface of the metal member with oxide film formation laser light (laser light for forming an oxide film). That is, the surface of the metal member is irradiated with laser light for a predetermined time in order to form an oxide film with a predetermined thickness that implements desired laser absorption. The metal member is then irradiated with heating laser light (laser light for heating) through the oxide film formed on the surface of the metal member. Since the metal member has enhanced laser absorption due to formation of the oxide film, the temperature of the metal member rises quickly, whereby efficient evaluation of the bonded state is achieved. It is known that laser absorption is saturated when the thickness of the oxide film is larger than a certain value. In the technique described in JP 2014-228478 A, the thickness of the oxide film at which laser absorption is saturated is set based on this knowledge, and the laser irradiation time is set so that the oxide film having this thickness can be formed.
However, it takes too long to form an oxide film with a certain thickness or more at which laser absorption is saturated as described in JP 2014-228478 A, which causes an increase in cost. Moreover, if the laser irradiation time is reduced in order to form an oxide film in a short time, the thickness of the oxide film is reduced. In the relationship with the thickness of a thin oxide film that has a near-zero thickness and can be formed by short-time irradiation with laser light, the laser absorption of the metal member has a periodic profile with maximal and minimal values appearing alternately as the thickness of the oxide film increases from zero. In this case, even a slight variation in thickness of the oxide film causes a great variation in laser absorption. Accordingly, forming a thin oxide film by the short-time laser irradiation is inexpensive, but stable laser absorption is less likely to be achieved.